1. Field of the Invention
The present invention relates to a driver for energizing data lines of a display panel, and more particularly to a display panel driver for displaying information on a display panel while correcting different light-emitting characteristics of red, green, and blue light-emitting elements of the display panel.
2. Description of the Related Art
In recent years, color display units employing electro luminescence (hereinafter abbreviated as “EL”) elements as self-emission elements have been put to practical use. FIG. 1 of the accompanying drawings is a block diagram of an EL display unit. As shown in FIG. 1, the EL display unit includes display panel 1 comprising a plurality of pixels 4 positioned at respective points of intersection between a plurality of data lines 2 and a plurality of scanning lines 3. Each of pixels 4 comprises EL element 9. EL elements 9 of pixels 4 that are selected by data lines 2 and scanning lines 3 emit light at an intensity according to drive voltages that are supplied over data lines 2.
The EL display unit also has data line driver 70 that is supplied with red input data Dr, green input data Dg, and blue input data Db and outputs drive voltages DV(1) through DV(k) to data lines 2. Data line driver 70 has drive controlling circuit 7 for controlling the timing to input and output data, and drive voltage generating circuit 71 for generating drive voltages to be output to data lines 2. The EL display unit further has scanning line driver 6 for controlling the scanning of scanning lines 3. In FIG. 1, each of input data Dr, Dg, Db is shown as comprising 4-bit data. However, each of input data Dr, Dg, Db may comprise 6-bit data, 8-bit data, or other data.
The EL elements have different light-emission characteristics for red, green, and blue. The drive voltages to be applied to the EL elements need to be processed for gamma correction depending on those different light-emission characteristics in order to display color images that are well balanced among red, green, and blue on the display panel. FIGS. 2(a) through 2(c) of the accompanying drawings show gamma correction curves for different colors. Specifically, FIG. 2(a) shows a gamma correction curve for red, FIG. 2(b) a gamma correction curve for green, and FIG. 2(c) a gamma correction curve for blue. Since display panels that employ EL elements need to carry out gamma correction according to the different gamma correction curves for red, green, and blue, the display panels need different gradation voltage generating circuits dedicated to red, green, and blue, respectively.
FIG. 3 of the accompanying drawings shows in block form conventional drive voltage generating circuit 71. As shown in FIG. 3, conventional drive voltage generating circuit 71 comprises red gradation voltage generating circuit 72 for being supplied with red power supply Vr and generating and outputting 4-bit voltages, i.e., 16 red gradation voltages Vr(0) through Vr(15), green gradation voltage generating circuit 73 for being supplied with green power supply Vg and generating and outputting 16 green gradation voltages Vg(0) through Vg(15), and blue gradation voltage generating circuit 74 for being supplied with blue power supply Vb and generating and outputting 16 blue gradation voltages Vb(0) through Vb(15). Red digital-to-analog converters (hereinafter referred to as “DACs”) 12 convert red gradation voltages Vr(0) through Vr(15) into gradation voltages corresponding to 4-bit input data Dr, and output gamma-corrected voltages through buffer circuits 15 as drive voltages to data lines 2. Green DACs 13 convert green gradation voltages Vg(0) through Vg(15) into gradation voltages corresponding to 4-bit input data Dg, and output gamma-corrected voltages through buffer circuits 15 as drive voltages to data lines 2. Similarly, blue DACs 14 convert blue gradation voltages Vb(0) through Vb(15) into gradation voltages corresponding to 4-bit input data Db, and output gamma-corrected voltages through buffer circuits 15 as drive voltages to data lines 2.
Details of the gradation voltage generating circuits and the DACs are disclosed in Japanese laid-open patent publication No. 2002-175060 (referred to as “first background art”), for example. As shown in FIG. 4 of the accompanying drawings, red gradation voltage generating circuit 72 divides the voltage supplied from red power supply Vr with resistors whose resistances have been selected for correction, generating and outputting red gradation voltages Vr(0) through Vr(15). Similarly, green gradation voltage gen-generating circuit 73 divides the voltage supplied from green power supply Vg with resistors whose resistances have been selected for correction, generating and outputting green gradation voltages Vg(0) through Vg(15). Blue gradation voltage generating circuit 74 divides the voltage supplied from blue power supply Vb with resistors whose resistances have been selected for correction, generating and outputting blue gradation voltages Vb(0) through Vb(15). Red DAC 12a has switches corresponding to the respective bits. Based on 4-bit red input data Dr, the switches are selectively opened and closed to select and output one of the gradation voltages. For example, if 4-bit red input data Dr represents (100), i.e., (8 h), then red DAC 12a selects and outputs gradation voltage Vr(8). Green DACs 13 and blue DACs 14 are also similarly constructed.
According to the first background art, however, since the gradation voltage generating circuits dedicated to red, green, and blue are required, there are required red, green, and blue power supplies, and also resistor strings having respective resistances selected for correction with respect to red, green, and blue. Consequently, data line driver 70 cannot be reduced in size, and cannot have its power consumption reduced.
Japanese laid-open patent publication No. 2001-92413 (referred to as “second background art”) discloses a conventional EL display unit which directly performs gamma correction on a video signal. FIG. 5 of the accompanying drawings shows in block form the conventional EL display unit according to the second background art. As shown in FIG. 5, video signal correcting circuit 82 is supplied with red input data Dr, green input data Dg, and blue input data Db, and corrects these input data in order to amplify or attenuate them based on corrective data stored in corrective memory 83. For example, video signal correcting circuit 82 corrects red input data Dr in order to amplify them, and outputs corrected red input data CDr to data line driver 81. Green input data Dg and blue input data Db are similarly corrected by video signal correcting circuit 82, which output corrected green input data CDg and corrected blue input data CDb to data line driver 81. Inasmuch as the red, green, and blue input data are gamma-corrected by video signal correcting circuit 82 and then input to data line driver 81, data line driver 81 needs to have a single gradation voltage generating circuit, and hence is made up of a reduced number of parts and has its power consumption reduced.
According to the second background art, however, when the input data are amplified by video signal correcting circuit 82, the number of gradation voltages is essentially increased to the extent that digital input data applied to DACs will exceed the number of convertible bits of the DACs. When this happens, the output gradation voltages produced in response to the input data are saturated, resulting in color irregularities on displayed images.